M. Lundh1*, I. Vassileva2, E. Dahlquist2 and E. Wackelgard1

1 Department of Engineering Sciences, The Angstrom Laboratory, P. O. Box 534, SE-751 21 Uppsala, Sweden

2 School of Sustainable Development of Society and Technology, Malardalen University, P. O. Box 883,

SE-721 23 Vasteras, Sweden

* Corresponding Author, Ma gdalena. Lundh @ angstrom. u u. se

Abstract

There are different methods to display energy use for hot water in households. The most direct method is to measure domestic hot water, but at present there is a lack of detailed measurements.

An alternative method is to model load curves. In this paper a model based on time-geographic data is evaluated by comparison to hot water use measured in two multi-family houses. Variations in demand are also studied to investigate possible model improvements. The results show that the modelled and measured daily energy demand agree rather well, both in magnitude and in distribution in time. Further investigations of measured data show that the distributions over the day coincide well for all investigated months, while a clear minimum in energy demand is found during vacation time in July and August. Comparison between different weekdays does not show any clear pattern. The findings can be used to further improve the model to include weekly and seasonal variations. The modelled profiles will later be used to investigate the influence of different draw-off pattern on solar heating utilization.

Keywords: DHW load profiles, model, time diary, measurement

1. Introduction

When performing solar heating simulations, rather simple hot water consumption profiles are usually used. The dynamic behaviour of household members is reduced to a schematic load profile, often assuming three draw-offs per day, not distinguishing between weekdays and weekend days or seasonal variations. To dimension a solar heating system correctly the actual hot water use in a building is however preferably applied. Different methods can be used to display energy use in households. The most accurate method to display the current use is to make measurements, but even measured data may not be useful to estimate future hot water demand. Moreover, it is costly to measure hot water use in detail and it often requires severe interference in the apartment if all different tap places are to be monitored separately. Few detailed hot water measurements have been performed so far, with the exception of a survey performed by the Swedish Energy Agency, ending in January 2008 [1]. Energy was however only measured in ten households, which is too few to be statistically significant.

Another approach to estimate hot water use is to model load curves. One method to generate hot water profiles was developed within IEA-SHC Task 26 on statistical means [2], which has been applied in several different studies. Models have also been developed for predicting domestic hot water load for Swedish conditions, such as Wollerstrand [3] based on Holmberg [4], using stochastic variables

derived from measurements. However, the existing models seldom focus on the influence of the individual household members and their behaviour. The method developed at Uppsala University, also reported in detail in [5, 6], and evaluated in this study, on the other side, is based on time-geographic studies by using data from so called time diaries to quantify hot water demand. It enables direct analysis of the correlation between actual behaviour and energy use. A certain energy demand can then be directly connected to a specific activity or user pattern, and since all household members record time diaries for the same days, the energy use can in addition be connected to a certain person.

1.1. Aim

The main aim of this study is to compare hot water user profiles modelled from time-use data with measurements in Swedish apartment households, but also to study how hot water demand varies in the measured households over the year. The results can then be used to further improve the model. The work is based on two different studies, one comprising measurements of domestic hot water use [7] and the other on development of a method for generating hot water load profiles [5, 6].

1.1. Outline

The two studies constituting the background of the comparison are first introduced. The comparison of the measured and modelled load profiles are then presented, as well as the variation in measured hot water demand during different periods of the year. Finally, the results, as well as possible improvements of the model, are discussed and concluding remarks given.

2. Background

In this study, two different approaches for generating hot water profiles have been compared; measurements in apartment households and modelling of hot water profiles from time-use data recorded by apartment household members.